1. Field of the Invention
The present invention relates to an antenna structure, more particularly to a quadri-filar helix antenna structure that can reduces the overall antenna size and greatly lowers its production and transportation costs.
2. Description of the Related Art
In recent years, as the wireless communication industry is blooming, the service of the satellite personal communication network (SPCN) available in the market gains more attentions, particularly the applications of handheld terminals (HHT). Therefore, people all over the world can communicate with each other by the handheld terminal through the satellite personal communication network, or the location of the handheld terminal holder can be located in a fast manner through the satellite personal communication network as to improve work efficiency and avoid unnecessary wastes of manpower, time and resources, and all these depends on the structure and design of the antenna of a handheld terminal.
Traditionally, an antenna used for receiving a satellite signal is usually deigned in a spiral structure. In other words, several radiating metal plates of the antenna are extended along a spiral path about the same axis to form a three-dimensional antenna. The antenna structure disclosed in the British Patent No. 2,258,776 belongs to this kind of antenna structure, and this patent makes use of a plurality of helical elements arranged around a common axis and extended along a spiral path to define a multiple-wire spiral antenna structure. Since such three-dimensional spiral antenna has an excellent receiving capability for the circularly polarized signals coming directly from the sky, and thus it is also called an antenna of the global positioning system (GPS) for receiving the coordinate positioning signals transmitted from a satellites group. In addition, the three-dimensional structure of this kind of antennas also fits an application as an omni-directional antenna for receiving vertically and horizontally polarized signals. However, one of the disadvantages of such three-dimensional antenna is that in certain applications, it is insufficiently robust, and cannot easily be modified to overcome this difficulty without a performance penalty.
In view of the shortcomings of the foregoing multiple-wire spiral antenna, many designs adopt a patch antenna such as the antenna installed on the outside of an aircraft fuselage for receiving the satellite signals under poor weather conditions. This kind of patch antennas comes with thin sheet radiating metal plates attached on an insulating material of the aircraft fuselage, but such patch antenna tends to have poor gain at low angles of elevation. With the efforts to overcome this disadvantage, antenna designers install a plurality of different batch antennas at different angles and different positions of the aircraft fuselage, and let the batch antennas be connected to the same receiver for receiving the satellite signals. This technique is expensive, not only due to the numbers of batch antennas required, but also due to the difficulty of combining the received signals and integrating all these batch antennas.
Please refer to FIG. 1 for the U.S. Pat. No. 6,424,316 issued to Leisten on Jul. 23, 2002, which effectively reduced the size of traditional quadri-filar antennas and designed a new quadri-filar antenna structure. The antenna comprises a cylindrical core 12 made of a ceramic material, four longitudinally extending antenna elements 10A, 10B, 10C, and 10D formed on a circumferential surface at an end proximate to the cylindrical core 12, and each antenna element 10A, 10B, 10C, and 10D is in the form of a metal plate, and a penetrating hole 14 is disposed along the radial direction of the center of the cylindrical core 12, and a metallic lining 16 is covered on the inner wall of the penetrating hole 14 and includes an insulator 17 inside. An axial feeder conductor 18 is installed at the central axis of the insulator 17, and a feeder structure is formed between the axial feeder conductor 18 and the metallic lining 16 to couple the feed line of a signal receiver (not shown in the figure) to each antenna element 10A, 10B, 10C, and 10D through the feed line.
The antenna structure further comprises a plurality of radial antenna elements 10AR, 10BR, 10CR, 10DR distributed on a surface at one end of the cylindrical core 12, and each radial antenna element 10AR, 10BR, 10CR, 10DR is substantially in the form of a metal plate being correspondingly and respectively coupled to one end of the antenna element 10A, 10B, 10C, 10D, such that one end of the antenna element 10A, 10B, 10C, 10D is coupled respectively with the feeder structure, and a common grounding conductor 20 is disposed on the circumferential surface at the other end proximate to the cylindrical body 12, and the common grounding conductor 20 is substantially in the shape of a circular ring being sheathed onto the circumferential surface at another end of the cylindrical body 12, and one end of the common grounding conductor 20 is coupled to another end of the antenna elements 10A, 10B, 10C, 10D, and the other end is extended to the surface of another end of the cylindrical body 12 to form a “sleeve balun” and coupled to the metallic lining 16.
Please refer to FIG. 1 for the structure of the antenna, wherein each antenna element 10A, 10B, 10C, 10D has a different length and a different shape, and any two of the antenna elements 10B, 10D are extended spirally along a meandering course on the circumferential surface of the cylindrical body 12, and thus its length is longer than the two antenna elements 10A, 10C that are extended spirally along a linear course on the circumferential surface of the cylindrical body 12.
From the issued patent of Leisten, it is known that the quadri-filar antenna uses a ceramic material with a high dielectric constant (∈r=36) for the core, and the electrical length is a half loop and the four spiral antenna elements 10A, 10B, 10C, 10D have a half wavelength, and thus it can greatly reduce the overall size of the traditional quadri-filar antenna. However, the manufacturing process involved is more complicated and requires copper plating, lithography, etching and laser trimming processes. Particularly the height of the sleeve balun must be controlled to the micro level in order to eliminate the unbalanced current and thus greatly increasing the manufacturing time, manpower, and cost.